It is important that reed switches used in a variety of electronic systems and devices such as automatic test equipment, process control equipment and data processing terminals should have very stable and consistent contact resistance over many millions of operations.
Although in most of the above applications the power handled by these switches is very low (less than 100 milliwatts) the very high number of operations during the expected life of the switches brings about gradual deterioration of the contact surface caused by minute electrical transients, mechanical erosion and electrochemical disturbances.
Dry reed switches are known for such applications which incorporate switch blades normally made of nickel/iron plated with a thin layer, about 2 um, of a suitable noble metal such as gold, palladium, rhodium or ruthenium.
During operation of these switches the plated noble metal is eroded by the arc formed on make and break, which causes a steady deterioration of contact resistance and limits the life of the switch.
In order to overcome such deterioration and obtain long-term contact resistance, the blades can be wetted by mercury. In general, only reed switches containing mercury are able to satisfy fully the requirements of contact resistance stability. In such mercury-containing reed switches the blades are provided with, or made of, a mercury-wettable material such as gold or palladium. In practice, the blades are generally mounted within the reed switch and a relatively large quantity of mercury is then introduced into the interior of the switch to form the mercury-wetted contact area.
These switches are difficult to make, expensive, generally have a pressurized hydrogen atmosphere, frequently are position-sensitive, and must have large separations between the contacting elements to permit the surface tension of mercury to be overcome during switch opening. The "all position" mercury-wetted reed switch avoids the position sensitivity of other mercury-wetted reed switches by careful control of the amount of mercury employed, but still suffers from the other drawbacks.
A known mercury-wetted reed switch comprises a pair of switch blades extending within a glass envelope and hermetically sealed therewith. Each switch blade has a spaded end portion the free ends of which overlap to form the switch contact. One of the spaded portions is either wholly or partially plated with a substance wettable by mercury, for example gold, the other blade being non-wettable by mercury except for a very small contact button welded to the blade.
To ensure that the mercury-wettable switch blade is provided with sufficient mercury to produce the contact resistance required, especially under high power applications, the mercury dosage is such as to provide a reservoir of mercury within the envelope.
The presence of such an excessive amount of mercury in the glass envelope has the disadvantage that the reed switch must be operated in a vertical position such that the mercury reservoir is located at the bottom of the capsule and therefore does not swamp the contact area between the blades, thus interferring with contact make and break operations.
Moreover the presence of liquid mercury in the envelope means that during the metal-to-glass sealing procedure, the mercury must be held at an extremely low temperature, down to around minus 50.degree. centigrade, so as to obviate the production of mercury vapour which would affect the seal.
In view of the deficiencies of prior art mercury-wetted switch contacts and switches containing them, it would be very desirable to have a reed switch possessing (1) long-term contact resistance stability equal to that of switches with mercury-wetted contacts, (2) the ability to operate in any orientation, (3) very low adhesion between the contact elements so that the switch can operate with low separation forces and with very small separations between the contacts, and (4) contact elements having unimpaired magnetic properties relative to dry reed switch contacts, so that the switch can operate in the usual way.